Harmonic flexible meshing gear device

ABSTRACT

The silk-hat-shaped flexible external gear (13) of a silk-hat flexible meshing gear device (11) comprises a body (22), a diaphragm (23) and a boss (25). Between the main portion of the body (22) and a curved portion (222) is formed a thinned portion (221) whose thickness is about 80% that of the main portion. The thickness of the diaphragm (23) is defined to be largest at the inner edge (point A), smallest at the middle portion (point B) and slightly smaller than the largest thickness at the outer edge (point C). To make the change in thickness smooth throughout, the contour is defined by use of a group of curved lines (223, 224). Providing the thinned portion (221). and defining the thickness of the diaphragm (23) in the foregoing manner relieves stress concentration at the inner and outer edges and smooths the stress distribution. As a result, the axial length and the outer diameter can be reduced to realize a compact silk-hat flexible meshing gear device.

TECHNICAL FIELD

This invention relates to a silk-hat flexible meshing gear deviceincluding a flexible external gear shaped like a silk hat. Moreparticularly, this invention relates to a silk-hat flexible meshing geardevice including a flexible external gear shaped like a silk hat whereinstress concentration in the flexible external gear can be relieved andthe flexible external gear can be reduced in outer diameter and axiallength.

BACKGROUND ART

A type of flexible meshing gear device with a flexible external gearshaped like a silk hat is known to the art. In this specification, thistype of flexible meshing gear device will be referred to as a "silk-hatflexible meshing gear device."

FIG. 6 is a vertical sectional view of the silk-hat-shaped flexibleexternal gear of a silk-hat flexible meshing gear device taken along aplane including the device axis. As shown in this figure, the flexibleexternal gear 1 comprises a cylindrical body 2, an annular diaphragm 3whose inner edge is continuous with the open proximal end of the body 2,and a thick annular boss 5 continuous with the outer edge of thediaphragm 3. External teeth 4 are formed integrally with the outersurface at the open distal end of the body 4 to run in thecircumferential direction.

This type of device is suitable for use in cases where a rotating memberor the like is passed through the device along the device axis 1a. Thisis because the provision of the diaphragm 3 of the flexible externalgear so as to extend radially outward from the end of the body 2facilitates accommodation of the rotating member in the internal spaceof the body 3.

Recent years have seen increased demand for more compact speed reducingmechanisms for use in small robots and the like. One way of respondingto this demand is to shorten the axial length of the silk-hat flexiblemeshing gear device. For this it is necessary to shorten the axiallength of the silk-hat-shaped flexible external gear. In the externalgears in general use, the ratio of the axial length of the flexibleexternal gear to the pitch circle diameter of its external teeth isaround 1:1. Making the axial length shorter than this increases theconing angle of the external gear.

In other words, the coning angle θ of the flexible external gear 1increases as shown in FIG. 5. As a result, excessive stressconcentration may occur at such points as the inner edge 3a and theouter edge 3b of the diaphragm 3 shown in FIG. 6. It may also becomeimpossible to maintain proper meshing of the external and internalgears. Because of this, a conventional flexible external gear merelyshortened in axial length is not useful in practical applications.

In the silk-hat flexible meshing gear device, since, as explained in theforegoing, the diaphragm 3 of the flexible external gear extendsradially outward from the end of the body 2, a rotating member or thelike can be passed through and accommodated in the internal space of thebody 3. However, this radially outward extension of the diaphragm 3gives the device a larger outer diameter than that of a cup-shapedflexible meshing gear device equipped with a cup-shaped flexibleexternal gear. The outer diameter of the device can be reduced byreducing the outer diameter of the diaphragm 3.

As shown by the arrows in FIG. 6, however, a large bending stressrepeatedly acts on the diaphragm 3 owing to deformation of the body 2flexed in the radial direction by a wave generator (not shown). If thediameter of the diaphragm 3 is reduced, therefore, the stress arising atthe inner edge 3a and outer edge 3b of the diaphragm 3 increases ininverse proportion. This leads to undesirable concentration of stress atthese inner and outer edge portions.

The applicant therefore earlier proposed a structure for relieving theconcentration of stress at the inner edge and outer edge of thediaphragm. This structure, which is disclosed in JU-B 3-118346, isobtained by thinning the diaphragm at its middle portion and thickeningit at both edge portions. However, this publication does not propose aspecific configuration for achieving the proposed thinning of thethickness at the middle portion of the diaphragm.

In view of the aforesaid point, an object of this invention is toprovide a silk-hat flexible meshing gear device wherein stressconcentration in the silk-hat-shaped flexible external gear can berelieved and the silk-hat-shaped flexible external gear can be reducedin axial length.

Another object of the invention is to provide a silk-hat flexiblemeshing gear device wherein stress concentration in the diaphragm isfurther relieved to enable a reduction in the device outer diameter.

Another object of the invention is to provide a silk-hat flexiblemeshing gear device wherein stress concentration in the silk-hat-shapedflexible external gear can be relieved and the silk-hat-shaped flexibleexternal gear can be reduced in both axial length and outer diameter.

DISCLOSURE OF THE INVENTION

This invention relates to a silk-hat flexible meshing gear device whichhas an annular rigid internal gear, a flexible external gear disposedinside the flexible external gear and a wave generator disposed insidethe flexible external gear which flexes the external gear in the radialdirection to mesh it partially with the rigid internal gear and rotatethe meshing position therebetween in the circumferential direction andwhose flexible external gear includes a cylindrical body formed withexternal teeth on its outer surface at one open end, an annulardiaphragm whose inner edge is continuous with another open end of thebody and an annular boss formed continuously with the outer edge of thediaphragm The invention achieves the aforesaid objects by defining thesectional shape of the flexible external gear in the following manner.

Specifically, the end portion of the body on the diaphragm side isformed with a curved portion curving perpendicularly to the device axisso as to continue into the inner edge of the diaphragm and the portionof the body adjacent to the start of the curved portion is constitutedas a thinned portion of less thickness than the portion adjacentthereto.

The thickness of the thinned portion is preferably about 0.8 of that ofthe adjacent body portion.

Forming the diaphragm side end of the body of the silk-hat flexiblemeshing gear with the thinned portion in this manner relieves theconcentration of stress at the inner edge and outer edge of thediaphragm and results in a smooth overall stress distribution.

In addition, the body length is preferably defined within theapproximate range of 0.2-0.7 of the pitch circle diameter of theexternal teeth of the external gear and the length of the external teethin the direction of the tooth trace is preferably defined within theapproximate range of 0.1-0.3 of the pitch circle diameter of theexternal teeth. Within these ranges, proper meshing of the external andinternal teeth can be maintained despite increase in the coning angleoccurring in the flexible external gear.

Further, the sectional shape of the diaphragm of the external gear in aplane including the device axis is preferably defined such that thethickness t(A) at the inner edge is greatest, the thickness t(B) at theportion substantially midway between the inner edge and the outer edgeis smallest, and the thickness t(C) at the outer edge falls between thethickness t(A) and t(B), namely, such that t(A)>t(C)>t(B).

In this case, in order to smooth the change in stress distribution fromthe inner edge of the diaphragm toward the outer edge thereof, it ispreferable to define the aforesaid sectional shape of the diaphragm ofthe external gear such that the contour of at least one surface of thediaphragm is defined by multiple curves to produce a smooth change inthickness.

Studies by the inventors showed that the relationship between thethickness t(A) at the inner edge and the thickness t(B) at the middleportion is preferably defined such that t(A)/t(B) falls in theapproximate range of 1.5-2.2.

The relationship between the thickness t(C) at the outer edge and thethickness t(B) at the middle portion is preferably defined such thatt(C)/t(B) falls in the approximate range of 1.4-2.0.

Defining the sectional shape of the diaphragm in this manner smooths thestress distribution in the diaphragm of the silk-hat flexible meshinggear from the inner edge toward the outer edge. It also prevents stressconcentration at the inner edge and the outer edge. As a result, theouter diameter of the diaphragm can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic sectional view of a silk-hat flexible meshing geardevice which is an embodiment of the invention.

FIG. 2 is a schematic front view of the silk-hat flexible meshing geardevice of FIG. 1 seen in the direction of the arrow in FIG. 1.

FIG. 3 is a vertical sectional view of a silk-hat-shaped flexibleexternal gear of the device of FIG. 1.

FIG. 4 is an enlarged view of a portion of the vertical section of theexternal gear shown in FIG. 3.

FIG. 5 is a diagram for explaining increase in coning angle caused byreducing the axial length of the flexible external gear.

FIG. 6 is a diagram for explaining a problem of the conventionalsilk-hat flexible meshing gear device.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be explained with reference to thedrawings in the following.

FIGS. 1 and 2 show the overall configuration of this embodiment of thesilk-hat flexible meshing gear device. The device 11 in these figures isconstituted of an annular rigid internal gear 12, a silk-hat-shapedflexible external gear 13 located inside the rigid internal gear 12, andan elliptical wave generator 14 fitted inside the flexible external gear13.

The flexible external gear 13 has a body 22, an annular diaphragm 23whose inner edge 23a is continuous with the open proximal end of thebody 22, and a thick annular boss 25 continuous with the outer edge 23bof the diaphragm 23. External teeth 24 are formed integrally with theouter surface of the open distal end of the body 22 to run in thecircumferential direction. The boss 25 is for attaching the device toanother object (not shown) so that body 22 and the diaphragm 23 arecantilevered from the boss 25.

The wave generator 14 is constituted of a hollow hub 14a, an ellipticalcam plate 14b fitted on the outer surface of the hub 14a, a ball bearing14c fitted on the outer surface of the cam plate 14b. The wave generator14 flexes the portion of the body 22 formed with the external teeth 24of the flexible external gear into elliptical shape. As a result, theexternal teeth at two portions located at opposite ends of the majoraxis of the ellipse mesh with inner teeth 12a of the rigid internal gear12. When the wave generator 14 is rotated about the device axis 11a, themeshing positions rotate in the circumferential direction. This rotationproduces a relative rotation between the flexible external gear 13 andthe rigid internal gear 12 proportional the difference in the number ofexternal and internal teeth. If, for example, the rigid internal gear 12is fixed and the wave generator 14 is used as a high-speed rotationalinput element, the flexible external gear 13 will become a reduced speedoutput element from which a rotational output of reduced speed can beobtained.

FIG. 3 is a vertical sectional view of the silk-hat-shaped flexibleexternal gear 13 of the external gear in a plane including the deviceaxis 11a. FIG. 4 is an enlarged view of a portion of the verticalsection of the external gear shown in FIG. 3.

The sectional shape of the respective portions as seen in a sectiontaken in a plane including the device axis 11a is follows. Starting fromthe opening 22a at the distal end of the body 22 of the flexibleexternal gear 13, the inner surface of the body 22 is defined by astraight line 22b lying parallel to the device axis 11a. The point 22cat the proximal end of the straight line 22b connects with an arc 22dcontinuing smoothly from the straight line 22b. At its other end,indicated as point A, the arc 22d continues smoothly into a straightline 231 lying perpendicular to the device axis 11a and defining therear surface of the diaphragm 23. At the outer edge indicated by pointC, the other end of the straight line 231 continues smoothly into an arc25a continuous with the rear surface of the boss 25. The other end ofthe arc 25a connects with a straight line 25b lying perpendicular to thedevice axis 11a.

The outer surface of the body 22 of the flexible external gear 13 isbasically defined by a straight line 22e lying parallel to the straightline 22b of the inner surface. As indicated earlier, the outer surfaceof the distal end of the body 22 is integrally formed with the externalteeth 24. The end of the straight line 22e continues smoothly into aconvex arc 22f having its center at ◯1. The arc 22f continues smoothlyinto a concave arc 22g having its center at ◯2. The concave arc 22gfurther continues smoothly into a concave arc 22h of smaller curvaturehaving its center at ◯3. The concave arc 22h continues smoothly into aconcave arc 22i of larger curvature having its center at ◯4. Thus theconcave arc 22g forms the proximal end portion of the body 22 with athinned portion 221 whose thickness is smaller than the thickness t(22)of the body 22 defined by the straight lines 22b, 22e.

The arc 22i continues smoothly into an arc 233 having its center at ◯5and defining the front surface of the diaphragm 23. Near the middle ofthe diaphragm 23, namely at a point B midway between point A and pointC, the arc 233 continues smoothly into an arc 234 of slightly largercurvature having its center at ◯6. The other end of the arc 234continues smoothly into a straight line 25c lying perpendicular to thedevice axis 11a and defining the front surface of the boss 25. Thus therear surface of the diaphragm 23 is defined by the straight line 231 andthe front surface thereof is defined by the two arcs 233, 234. The twoarcs 233, 234 connect at the point B at the middle of the diaphragm 23.The smallest thickness of the diaphragm 23 defined by the straight lineand arcs is the thickness t(B) at the middle point B. The greatestthickness is the thickness t(A) at point A at the inner edge of thediaphragm 23. The thickness t(C) at point C at the outer edge of thediaphragm 23 is slightly larger than the thickness t(A).

In the body 22 of this embodiment, therefore, the sectional shape of themain portion of the body 22 is defined by the straight line 22e and thestraight line 22b. The portion continuous with this portion is definedby the concave arc 22g and the straight line 22b as the thinned portion221. In addition, the portion adjacent to the thinned portion 221 isdefined as a curved portion 222 defined by the concave arc 22i and theconvex arc 22d to be continuous with the inner edge of the diaphragm.

In the flexible meshing gear device 11 of this embodiment, since thebody 22 is formed with the thinned portion 221 and the diaphragm 23 isdefined to have the sectional shape described in the foregoing, thedistribution of the stress produced in the diaphragm 23 during operationis smoother and more uniform than in the prior art. Moreover, theconcentration of stress at the inner edge and the outer edge isthoroughly relieved. Since the stress produced in the diaphragm 23 canbe reduced in this way, the length L(22) of the body 22 can beshortened. The outer diameter of the diaphragm 23 can also be madesmaller than in the prior art. In other words, the outer diameter of thedevice indicated as D in FIG. 3 can be reduced.

While the thinned portion 221 is formed on the outer surface of the body22 in this embodiment, it is possible instead to form a similar thinnedportion on the inner surface of the body 22 or to form a thinned portionby defining both the outer surface and the inner surface with concavelines.

Tests and the like conducted by the inventors showed that the thicknesst(221) of the thinned portion 221 of the body 22 is preferably set atabout 80% of the thickness t(22) of the body 22.

On the other hand, tests conducted by the inventors showed that when thethicknesses at point A and point C are set within the following rangesrelative to the smallest thickness t(B), the stress distribution becomessmooth between the inner edge and the outer edge of the diaphragm 23 andthe concentration of stress at the inner edge and the outer edge isrelieved.

    1.5<t(A)/t(B)<2.2

    1.4<t(C)/t(B)<2.0

In this embodiment, two arcs 233, 234 are used to define the thicknessof the diaphragm 23. However, it is also possible to use three or morearcs to define the thickness of the diaphragm 23. While the rear surfaceof the diaphragm 23 is defined by the straight line 231, it can insteadbe defined by curved lines and the front surface be defined by astraight line. It is also possible to define a sectional shapesatisfying the aforesaid conditions by defining both surfaces of thediaphragm 23 with curved lines.

The coning of the flexible external gear 13 in the silk-hat flexiblemeshing gear device 11 will now be considered. The flexible externalgear 13 is repeatedly flexed into elliptical shape by the wave generator14 fitted therein. To reduce the coning force acting on the ball bearing14c of the wave generator 14 as a result of this deformation, i.e.coning, it is preferable to shorten the length (tooth length) L(24) ofthe external teeth 24 in the tooth trace direction. When the toothlength L(24) is shortened, the axial length L(22) of the body 22 canalso be shortened by a corresponding amount. In other words, a flexiblemeshing gear device of short axial length can be realized.

When the axial length is shortened, however, the coning angle θ of theflexible external gear 13 increases as shown in FIG. 5. As a result,increased stress is produced in the diaphragm 23.

It has been found, however, that when the thicknesses of the differentportions of the flexible external gear 13 are defined as in the device11 of the embodiment described in the foregoing, excessive stressconcentration does not arise and a smooth distribution of the producedstress can be achieved even if the axial length is shortened.

Tests and the like conducted by the inventors showed that the lengthL(22) of the body 22 of the flexible external gear 13 is preferably setin the approximate range of 20-70% of the opening diameter of theexternal teeth, namely the pitch circle diameter D(P) of the externalteeth. It was further found that from the practical viewpoint the lengthL(24) of the external teeth in the tooth trace direction shouldpreferably be set in the approximate range of 10-30% of the pitch circlediameter P(D).

INDUSTRIAL APPLICABILITY

As explained in the foregoing, in the silk-hat flexible meshing geardevice according to this invention, the end of the body of thesilk-hat-shaped flexible external gear on the side of the diaphragm isformed with a curved portion curving perpendicularly to the device axisso as to continue smoothly into the inner edge of the diaphragm and theportion of the body adjacent to the start of the curved portion isconstituted as a thinned portion of less thickness than the portionadjacent thereto. Forming the thinned portion at this location relievesthe concentration of stress at the inner edge and outer edge of thediaphragm and results in a smooth overall stress distribution. As aresult, it is possible to use a shorter flexible external gear than inthe past.

The body length is set within the approximate range of 0.2-0.7 of thepitch circle diameter of the external teeth of the external gear and thelength of the external teeth in the direction of the tooth trace is setwithin the approximate range of 0.1-0.3 of the pitch circle diameter ofthe external teeth. As a result, proper meshing of the external andinternal teeth can be maintained despite increase in the coning angle ofthe flexible external gear.

In the silk-hat flexible meshing gear device according to the invention,the thickness of the diaphragm of the silk-hat-shaped flexible externalgear is defined such that the middle portion is thinnest, the inner edgeis thickest and the outer edge is thinner than the inner edge butthicker than the middle portion and such that this thickness pattern isobtain by smooth thickness variation from the inner edge to the outeredge.

In accordance with this invention, therefore, concentration of stress atthe inner edge and the outer edge of the diaphragm can be relieved anddistribution of the stress produced in the diaphragm can be madeuniform. As a result, the outer diameter of the diaphragm can also bemade smaller than in the prior art. Moreover, since excessiveconcentration of stress at the inner and outer edges of the diaphragmcan be avoided even if the length of the body is shortened, a flexibleexternal gear of short axial length can be realized.

We claim:
 1. A silk-hat flexible meshing gear device which has anannular rigid internal gear, a flexible external gear disposed insidethe rigid internal gear and a wave generator disposed inside theflexible external gear which flexes the external gear in a radialdirection to partially mesh it with the rigid internal gear and rotate ameshing position therebetween in a circumferential direction and whoseflexible external gear includes a cylindrical body formed with externalteeth on its outer surface at one open end, an annular diaphragm whoseinner edge is continuous with another open end of the body and anannular boss formed continuously with an outer edge of the diaphragm,characterized in thatthe end of the body of the flexible external gearon the diaphragm side is formed with a curved portion curvingperpendicularly to a device axis so as to continue smoothly into theinner edge of the diaphragm and a portion of the body adjacent to thestart of the curved portion is constituted as a thinned portion of lessthickness than a portion adjacent thereto.
 2. A silk-hat flexiblemeshing gear device according to claim 1, wherein the thickness of thethinned portion is set at about 0.8 of the thickness of the adjacentportion of the body.
 3. A silk-hat flexible meshing gear deviceaccording to claim 1, wherein the body has a length set within anapproximate range of 0.2-0.7 of a pitch circle diameter of the externalteeth of the flexible external gear.
 4. A silk-hat flexible meshing geardevice according to claim 1, wherein the external teeth have a length ina tooth trace direction set within an approximate range of 0.1-0.3 ofthe pitch circle diameter of the external teeth.
 5. A silk-hat flexiblemeshing gear device according to claims 1, wherein a sectional shape ofthe diaphragm of the flexible external gear in a plane including adevice axis is defined such that a thickness t(A) at the inner edge isgreatest, a thickness t(B) at a portion substantially midway between theinner edge and the outer edge is smallest, and a thickness t(C) at theouter edge falls between the thickness t(A) and t(B), namely, such thatt(A)>t(C)>t(B).
 6. A silk-hat flexible meshing gear device according toclaim 5, wherein the sectional shape of the diaphragm of the flexibleexternal gear in a plane including the device axis is such that acontour of at least one surface of the diaphragm is defined by multiplecurves to produce a smooth increase in thickness from the middle portiontoward the inner edge and the outer edge.
 7. A silk-hat flexible meshinggear device according to claim 5, wherein a relationship between thethickness t(A) at the inner edge and the thickness t(B) at the middleportion is defined such that t(A)/t(B) falls in an approximate range of1.5-2.2.
 8. A silk-hat flexible meshing gear device according to claim5, wherein a relationship between the thickness t(C) at the outer edgeand the thickness t(B) at the middle portion is defined such thatt(C)/t(B) falls in an approximate range of 1.4-2.0.
 9. A silk-hatflexible meshing gear device which has an annular rigid internal gear, aflexible external gear disposed inside the rigid internal gear and awave generator disposed inside the flexible external gear which flexesthe external gear in a radial direction to partially mesh it with therigid internal gear and rotate a meshing position therebetween in acircumferential direction and whose flexible external gear includes acylindrical body formed with external teeth on its outer surface at oneopen end, an annular diaphragm whose inner edge is continuous withanother open end of the body and an annular boss formed continuouslywith an outer edge of the diaphragm, characterized in thata sectionalshape of the diaphragm of the flexible external gear in a planeincluding a device axis is defined such that a thickness t(A) at theinner edge is greatest, a thickness t(B) at a portion substantiallymidway between the inner edge and the outer edge is smallest, and athickness t(C) at the outer edge falls between the thickness t(A) andt(B), namely, such that t(A)>t(C)>t(B).
 10. A silk-hat flexible meshinggear device according to claim 9, wherein the sectional shape of thediaphragm of the flexible external gear in a plane including the deviceaxis is such that a contour of at least one surface of the diaphragm isdefined by multiple curves to produce a smooth increase in thicknessfrom the middle portion toward the inner edge and the outer edge.
 11. Asilk-hat flexible meshing gear device according to claim 9, wherein arelationship between the thickness t(A) at the inner edge and thethickness t(B) at the middle portion is defined such that t(A)/t(B)falls in an approximate range of 1.5-2.2.
 12. A silk-hat flexiblemeshing gear device according to claim 9, wherein a relationship betweenthe thickness t(C) at the outer edge and the thickness t(B) at themiddle portion is defined such that t(C)/t(B) falls in an approximaterange of 1.4-2.0.